World Bank researchers have been trying to assess the extent of extreme poverty across the world since 1979 and more systematically since the World Development Report 1990, which introduced the dollar-a-day international poverty line. From the beginning, the idea was to measure income poverty with respect to a demanding line which, first, reflects the standards of absolute poverty in the world’s poorest countries and, second, corresponded to the same real level of well-being in all countries. The first requirement led researchers to anchor the international poverty line on the national poverty lines of very poor developing countries. And the second requirement led them to use purchasing power parity exchange rates (PPPs) – rather than nominal ones - to convert the line into the US dollar and, more importantly, into the currencies of each developing country.
A fascinating feature of purchasing power parity (PPP) is more people hold an opinion on it than know what it means. This was in ample display last week, when the Global Office of the International Comparison Program (ICP), hosted by the World Bank, announced the latest PPP data for the world, pertaining to 2011.
Putting aside complexities, PPPs may be viewed as an estimate what one US dollar can buy in different countries. In case a dollar in Ghana can buy three times what it can buy in the United States, then a person who earns 1,000 dollars each month in Ghana is said to earn 3,000 in terms of ‘PPP-adjusted dollars’.
In his Inquiry into the Nature And Causes of the Wealth of Nations Adam Smith pointed to the social-inclusion role of a linen shirt in 18th century Europe:
“A linen shirt … is, strictly speaking, not a necessary of life. The Greeks and Romans lived, I suppose, very comfortably though they had no linen. But in the present times, through the greater part of Europe, a creditable day-labourer would be ashamed to appear in public without a linen shirt, the want of which would be supposed to denote that disgraceful degree of poverty which, it is presumed, nobody can well fall into without extreme bad conduct.”
This passage has often been used to justify the view that poverty is not absolute but relative—that certain socially-specific expenditures are essential for social inclusion, on top of basic needs for nutrition and physical survival.
The way this idea is implemented in practice is to set a “relative poverty line” that is a constant proportion of average income for the country and date in question. That is how poverty is measured in most of Western Europe. By contrast, poverty measures in developing countries have almost invariably used absolute lines, which aim to have a fixed real value over time. The World Bank’s international “$1 a day” poverty lines also aim to be absolute lines across countries, using purchasing power parities from the International Comparison Program.